Revealing the role of double-layer microenvironments in pH-dependent oxygen reduction activity over metal-nitrogen-carbon catalysts

A standing puzzle in electrochemistry is that why the metal-nitrogen-carbon catalysts generally exhibit dramatic activity drop for oxygen reduction when traversing from alkaline to acid. Here, taking FeCo-N6-C double-atom catalyst as a model system and combining the ab initio molecular dynamics simulation and in situ surface-enhanced infrared absorption spectroscopy, we show that it is the significantly distinct interfacial double-layer structures, rather than the energetics of multiple reaction steps, that cause the pH-dependent oxygen reduction activity on metal-nitrogen-carbon catalysts. Specifically, the greatly disparate charge densities on electrode surfaces render different orientations of interfacial water under alkaline and acid oxygen reduction conditions, thereby affecting the formation of hydrogen bonds between the surface oxygenated intermediates and the interfacial water molecules, eventually controlling the kinetics of the proton-coupled electron transfer steps. The present findings may open new and feasible avenues for the design of advanced metal-nitrogen-carbon catalysts for proton exchange membrane fuel cells.

Supplementary Figure 2. Representative snapshots of the interfacial structures at ORR potentials on the bare FeCo-N6-C electrode surface for (a) alkaline system and (b) acid system.
The K + , OH -, H3O + and F -are colored with purple, yellow, green and cyan, respectively (similarly hereinafter).The brown dashed lines represent the hydrogen bonds.
In this work, the definition and adjustment of the electrode potentials corresponding to the experimental ORR conditions are based on the interfaces with *O2 (Fig. 1 in the text), rather than the clean interfaces where no adsorbates exist as shown in Supplementary Fig. 2.This is mainly for two reasons.First, it can be realized that there should always exist oxygen-containing intermediates (e.g., *OOH, *OH, *O) along the whole ORR process.Secondly, due to the electron-withdrawing effect, the interfaces with these oxygen-containing intermediates often possess distinctly higher potentials comparing to that without any oxygen-containing intermediates.For example, the potentials of interfaces shown in Fig. 1b and Supplementary Fig. 2b are 0.88 V and 0.58 V, respectively.It means that under the ORR condition, the interfacial microenvironment will change from the bare system to the oxygen-containing adsorbed systems when the electrode potential is controlled at a same value.In other words, if we define the electrode potential using the interface where no adsorbate exists, the potentials of the subsequent surfaces with adsorbates should also be readjusted again.Therefore, we define the electrode potential using the interface with *O2 intermediate, which is the first reaction intermediate state along ORR process.The PZTCs for Fe-N4-C and Co-N4-C electrodes with *O2 (Supplementary Fig. 4c,d) are calculated as 0.17 V and 0.37 V vs SHE.The calculated work functions (Φ).

Supplementary
Due to the limitation of computational cost when the active site density (SD) in model is similar to experimental values, the consideration of SD in the model is often ignored in most of the current ab initio simulation studies.However, evaluating the active site density in the model and its influence on the potential calculation and the results that are obtained based on the fairly small model (namely high SD) is fairly significant.
Therefore, we have estimated the SD in the synthesized FeCo-N6-C catalyst and performed additional AIMD simulations for the interfaces with SD closer to the experimental value.Firstly, according to the composition measurement of the as-prepared FeCo-N6-C sample by ICP-AES test 3 , the experimental SD is estimated as ~1.1×10 20 site/g, which agrees well with other reported values from 3×10 19 site/g to 2.8×10 20 site/g [4][5][6][7] .By contrast, the SD of the AIMD model shown in Figure 1 of the text, Supplementary Fig. 1 and Supplementary Fig. 5a,d, which is calculated as 9.2×10 20 site/g, is obviously higher than the experimental SD of prepared FeCo-N6-C catalyst.
To evaluate the influence of the SD in interface model on the potential calculation and the results that are obtained based on the fairly small model (namely high SD), we have established two larger FeCo-N6-C models (Supplementary Fig. 5b,c The distribution of total cosα displays a series of peaks with much higher amplitudes within ~5.6 Å at acid interface while only a fairly weak and narrow peak within ~3.5 Å at alkaline interface (Supplementary Fig. 14a).This implies that despite the presence of *OHAd-OS and *OAd-OS, the interfacial water molecules are still orientated orderly in the form of O-down configuration due to the positively charged electrode surface in acid media, while disorderly in alkaline media.
Correspondingly, the O atom of end-on adsorbed O2 that points to the solution can form the hydrogen bonds with the H atoms of interfacial water in alkaline media (Supplementary Fig. 14b).Furthermore, it can be seen that the O-O bond length of adsorbed O2 molecule at alkaline interface is obviously larger than that at acid interface (1.36 Å vs 1.29 Å), due to the assistance of hydrogen bonds formed with interfacial water (Supplementary Fig. 14c,d).Notably, compared with the O-O bond length (1.90 Å) at the alkaline interface without oxygenated spectators (Figs.The distribution of total cosα displays a series of peaks with much higher amplitudes within ~7.1 Å at acid interface while only a fairly weak and narrow peak within ~3.5 Å at alkaline interface (Supplementary Fig. 15a).This implies that despite the presence of *OHAx-OS and *OAx-OS, the interfacial water molecules are still orientated orderly in the form of O-down configuration due to the positively charged electrode surface in acid media, while disorderly in alkaline media.
Correspondingly, the O atoms of adsorbed O2 can form hydrogen bonds with the H atoms of interfacial water in alkaline media (Supplementary Fig. 15b).Furthermore, as depicted in reaction intermediate states are shown in Supplementary Fig. 16c,d.It can be noted that due to the existence of *OHAd-OS and *OAd-OS, the Co atom with relatively weak oxygenophilicity has been poisoned and thereby the synergistic effect between adjacent metal active sites is broken.
Therefore, at both alkaline and acid interfaces with the existence of Ad-OS, the adsorbed O2 molecules in the end-on configurations do not dissociate (Fig. 5a,b in the text and Supplemental Fig. 16), resulting in the formation of the *OOH intermediates.At this time, it can be realized that the FeCo-N6-C double atom catalyst is similar to the Fe-N3O-C single atom catalyst to a certain extent.
Furthermore, Supplementary Fig. 16e shows that in the presence of *OHAd-OS and *OAd-OS, the third PCET step (IV→V) is the PDS of alkaline ORR; while for acid ORR, the PDS is the first PCET step (II→III).Notably, the free energy change of PDS at acid interface is still much smaller than that at alkaline interface (0.47 eV vs 1.39 eV), contradicting the experimental truth that the ORR activity in alkaline is superior to that in acid.Furthermore, Supplementary Fig. 17e shows that in the presence of *OHAx-OS and *OAx-OS, the fourth PCET steps (V→I) are the PDS of both alkaline and acid ORR.Notably, the free energy change of PDS at acid interface is still smaller than that at alkaline interface (0.86 eV vs 0.96 eV), contradicting the experimental truth that the ORR activity in alkaline is superior to that in acid.
state is always much higher in free energy than that of TS for alkaline interface, which unequivocally demonstrates that the PECT reaction at acid interface is very sluggish.In other words, the second correction term ( −   ) is performed, in which  is the target potential (1.0 V vs RHE) and   is the actual potential of AIMD simulated acid or alkaline system (viz.0.88 V or 1.10 V vs RHE), so as to compare the reaction free energy diagrams in alkaline and acid at the same electrode potential.

Figure 3 .Supplementary Figure 4 .
Comparison of the bond lengths of adsorbed O2 at alkaline and acid interfaces among the whole 10 ps AIMD simulations.(a,b) Representative snapshots of Fe-N4-C/water and Co-N4-C/water interfaces.(c,d) Representative snapshots of Fe-N4-C/water and Co-N4-C/water interfaces with *O2.(e,f) Pourbaix diagram showing the pH dependence of the ORR reaction potential (1.0 V vs RHE is used here), the potential of zero free charge (PZFC) and potential of zero total charge (PZTC) for Fe-N4-C/water and Co-N4-C/water systems.The PZFCs for Fe-N4-C and Co-N4-C electrodes (Supplementary Fig.4a,b) are calculated as -0.71 V and -0.34 V vs SHE, which are similar to the values calculated by Chan and Liu 1,2 .

Supplementary Figure 12 .Supplementary Figure 13 .
Fig. 4b for examples) is very likely to result in the existence of oxygenated spectators.Such scenario has been considered in this work.To determine what exactly the surface oxygenated spectators are in alkaline and acid medias under experiment ORR potentials, the *OH oxygenated species on the Fe-Co bridge sites are evaluated, as shown in the left panels of Supplementary Fig.13a,b.At alkaline interface, the *OH occupying the Fe-Co bridge site remains stable throughout the 18 ps AIMD simulation (Supplementary Fig.13a), which implies that the oxygenated spectator should be *OH (termed as *OHS).While at acid interface, the *OH occupying the Fe-Co bridge site is oxidized spontaneously and turns into a *O species and a H3O + cation after merely 0.2 ps AIMD 1a and 2d), the adsorbed O2 molecule in the end-on configuration at the alkaline interface with the existence of *OHAd-OS does not dissociate.Supplementary Figure 15.(a) Distribution profiles of water dipole orientations along the surface normal direction at acid and alkaline interfaces when the *OHAx-OS and *OAx-OS are considered.The insets show that α is defined as the angle between the vector of water dipole ( ̂) and the surface normal ( �).(b) Radial distribution functions between the O atom of adsorbed O2 molecule and the H atoms of interfacial water.(c) The extraction of O-O bond length of the adsorbed O2 molecule during the 10 ps AIMD product simulations for alkaline and acid systems.(d) Statistical distributions of the O-O bond length of adsorbed O2 molecule at acid and alkaline interfaces.
acid medias when the *OHAx-OS and *OAx-OS (marked by yellow color) are considered.The H2O marked by red color represent the water molecules which locate in the solution and provide proton to surface oxygenated intermediates, while the H2O marked by green color mean the reaction products.(c,d) Representative snapshots of the interfacial structures along the ORR processes in (c) alkaline and (d) acid medias.(e) Free energy diagrams for ORR at alkaline and acid interfaces for U = 1.0 V vs RHE.The determined ORR pathways at alkaline and acid interface when the Ax-OS are considered are show in Supplementary Fig. 17a,b, and the corresponding interface structures of various reaction intermediate states are shown in Supplementary Fig. 17c,d.It can be noted that the O2 molecule adsorbs on the Fe-Co bridge site at alkaline interface with the existence of *OHAx-OS, thus leading to the dissociative mechanism of ORR.While, due to the strong interaction between *OAx-OS and Fe-Co bridge site, the interaction between Co atom with relatively weak oxygenophilicity and O atom of adsorbed O2 has been greatly weakened, thus resulting in that the Co atom cannot adsorb O2 to form the Fe-Co bridge adsorption configuration.In other words, at acid interfaces with the existence of *OAx-OS, the adsorbed O2 molecule is in the end-on configuration, resulting in the associative mechanism of ORR.
Figure 20.(a-c) The interface structures (upper panel) and corresponding close-up (lower panel) of the initial state (IS), transition state (TS) and final state (FS) in one typical slow-growth simulation for the first PCET reaction of acid ORR.At acid interface, in the process from IS to TS, the reactive water molecule first gradually approaches the O atom on Fe site with maintaining the O-down configuration, and then its orientation is flipped to the 'one-H-down' configuration in order to provide the H atom to the O atom of adsorbed O2 molecule.Due to the strong electrostatic interaction to interfacial water molecule exerted by the positively charged FeCo-N6-C electrode, the flip of water dipole orientation contributes to a fairly high barrier.The following process from TS to FS corresponds to the bond formation between the O atom of O2 on Fe site and the H atom of reactive water molecule, and meanwhile the generated OH -anion turns into water molecule by receiving a H atom from another water molecule that locates further away from the electrode.